Thermal treatment of powdered or granular material

ABSTRACT

A device for the thermal treatment of powdered or granular materials, including a container, a plurality of substantially circular trays mounted one above the other in said container for the sequential transfer of material downwardly from an outlet region of one tray to an inlet region of a next lower tray, a temperature control system for selectively controlling the temperatures of the trays, and a vibrational drive vibrating the trays selectively and adjustably for moving material on the trays from their inlet regions to their outlet regions with preselected material layer depths. A method for drying granular or powedered polymer material having a crystallization temperature region below 200* C, in a drying chamber maintained at a pressure of less than one torr and equipped with a material-receiving surface, which method includes forwarding the material on the surface by vibration in a layer of less than a predetermined layer depth, for preventing agglomeration when the material is in its crystallization temperature region, and bringing the temperature of the material into its crystallization temperature region.

' United States Patent [191 Bettermann et al.

[ July 3,1973

[ THERMAL TREATMENT OF POWDERED OR GRANULAR MATERIAL [73] Assignee:Leybold-Heraeus-Verwaltung Gmbll, Koln-Bayental, Germany [22] Filed:Oct. 4, 1971 [21] Appl. No.: 185,949

[30] Foreign Application Priority Data Oct. 2, 1970 Germany P 20 48487.1 Oct. 2, 1970 Germany P 20 48 494.0

[52] US. Cl 34/147, 34/92, 34/164, 34/ 178 {51] Int. Cl. F26!) [58]Field of Search 34/15, 92, 39, 147, 34/171, 178,164, 51

[56] References Cited UNITED STATES PATENTS 2,523,552 9/1950 Birdseye34/164 2,935,763 5/1960 Newman et a1... 34/92 2,946,429 7/1960 Carrier34/147 2,983,051 5/1961 Zimmermann et al. 341164 3,154,621 10/1964 Alban34/164 3,574,951 4/1971 Oetjen et al. 34/92 Primary Examiner-Carroll B.Dority, Jr. Assistant Examiner -Larry l. Schwartz Attorney-George l-l.Spencer et al.

( 5 7 ABSTRACT A device for the thermal treatment of powdered orgranular materials. including a container, a plurality of substantiallycircular trays mounted one above the other in said container for thesequential transfer of material downwardly from an outlet region of onetray to an inlet region of a next lower tray, a temperature controlsystem for selectively controlling the temperatures of the trays, and avibrational drive vibrating the trays selectively and adjustably formoving material on the trays from their inlet regions to their outletregions with preselected material layer depths.

temperature of the material into its crystallization temperature region.

11 Claims, 9 Drawing Figures [451 July 3,1973

United States Patent [191 Bettermann et al.

1 THERMAL TREATMENT O rowers 0s GRANULAR MA MAL BACKGROUND OF HINVENTION 'hs m Q eig ter m- An earl er ti -vis he mally t ating p de eqor sranala materiel i dis lose n U. s. Ba 'NQ- applie tan iisr- Nh- 16697. filed Ma hi 1 .9.1 Hams l ilehhers t r a Va uum D ye t FlowshleMatsthil lathe slavish. m ia cha s to i msve thrhhsh alw s with the samelqcity- The d pth of th his! of ma rial hi v hs h ough e svis i alwaysthe am Als a of the r ing h fas s in the evis exhibit th m e p r ur durhe the? ha mater a s flow g throu In the drying of crystallizing,granular or powdered pla ics. which' ay, equen y b a or Qx ssh s n it thr e ha g th t e po ders?! 9? g hhhlh! mat r a m y r a ts c y a za i ntemp r ure s in has, lathe ihh d hoh o t eat i sh sd or d ihs- In th eio p wd red stanhla matss sss m ss s v y qh lsl s ha l i e y arge chunksof material result. These chunks only very Posvly W9 asidihsnahy. clog hu s separatistrys sll ihsi anu a p wde ed shs ss ha beentreatedbatch-wise in a gyratory vacuum d er. W th?! @FYFPS are i dr s ll, r cining o a ners. which ta e a u a reclining ax Pa ve out e at the r e t axish hea ing meqihhi dish hi o gha dou e hell. The ea xc anging surfacea l n e at onsh p to the a o m is char ed r und. 1.0.9 kilograms). o thao s dr n time isuhue, to h it in w tch ne RQF IlQH mpo ary sto age of te P duc requ r d be s"? and after the thermal treatment. This isespecially problem when the granular material must not come in n s w thh or xy e hl d y p duct f he yp d sr hedy n a gym: tory dryer to an endmoisture content of about Q. l percent, total batch times of 16 it? 2 4.hours. re ult. These times n e SQmeW-hat h r ened. b buildin -in tr cwen a ge the o l h a ex hangi g. surfac and. to Pr v de ncr ased m v me hepmduc Y t. the, t h imssare i ly duc o ar n 2 *9. 16. ham

And, these batch times are only valid forplastics that do not reachtheir crystallization temperature region during thedrying. In the caseof plastics with lowcrys; l zatio e ps a h. e sihn ne. s. f rssd m p atewi h w ea nsemedih mpera ures. n 1119 to. p vehtposrly ry n sslome ates;he bat h d ying times are naturally correspondingly increased.

The gyratory dryer is furthermore very space-y a d he inc unker an po te u pment ann he hiiih tsdh ,1 perati n hste a PI55 t s are on n ous y.ryins h a. syrat ryus p es nts. i turb ng r hphoh f" he pr du flown.ddit on isadva ta e lies h h t af e ash batch, an intensive andfrequently difficult cleaning of the dryer must be carried out.

Also used for thermally treating crystallizing, granular or powderedplastics have been continuous processes utilizing the fluidized-bedconcept. In these processes, gas flows upwardly through a column ofproduct. Using water-vapor-partial-pressure and tempera ture of the gas,material and heat transport is carried out. Along with the continuousoperation of these processes, it has been possible to significantlyreduce drying times. These processes operate at normal pressure; air ornitrogen is used as gas. The gas is dried and given the propertemperature and is usually re-circulated and continuously regenerated.When a plastic tends to discolor at higher temperatures due to theinfluence of oxygen, nitrogen is used as gas.

The advantage of the good heat transfer characterizing thesefluidized-bed processes brings with it, however, a disadvantage. Due tothe fluidizing of the solid particles by the gas and especially whenoperating continuously, the single particles experience differentresideuce times in the fluidized column. Certain particles pass from theinlet of the column to the outlet on the fastest and shortest path,while other particles are apt to rise and fall within the column severaltimes before reaching the outlet.

Another disadvantage inherent in the fluidizing is a wearing of theindividual particles. Product is reduced in size and the resulting dustmust be seperated from the gas.

in general, fluidized-bed systems are, together with the above-describeddisadvantages, more demanding of maintenance, and they present verysignificant difficulties for the provision by automatic control of astable- P htiQn- SUMMARY OF THE INVENTION An object of the invention,therefore, is to provide an improved device of the type disclosed in U.S. Pat. No. application Ser. No. 16,667, to dry crystallizing, granularv or powdered polymer material with far better results than have yetbeen obtained with gyratory dryers and fluidized-bed dryers. 7 Anotherobject of the invention is to provide a device for the thermaltreatment, especially vacuum drying, of powdered or granular material,where the charged material is moved by vibration on a plurality ofsequentially arranged treatment surfaces, and where it is possible toset in simple manner the conditions required for an optimum thermaltreatment.

Yet another object of the invention is to provide a process for dryinggranular or powdered polymer material having a crystallizationtemperature region below 200 C to a final moisture content below 0.1weight? percent, which process escapes the disadvantages asahovevdescribed, and in particular escapes the troublesome agglomerationof the processes of the past.

These as well as other objects which will become apparent in thediscussion that follows are achieved, ac-

cording, to the present invention, by a device including a container, aplurality of substantially circular trays mounted one above the other inthe container for the sequential transfer of material downwardly from anoutlet region of one tray to an inlet region of a next lowertray, ameans for selectively controlling the temperatures of the trays, andmeans for vibrating the trays selectively and adjustably for movingmaterial on the trays from their inlet regions to their outlet regionswith preselected material layer depths; and by a process wherein thelayer depth of crystallizing, powdered or granular material iscontrolled at less than a predetermined thickness, for preventingagglomeration when the material is in its crystallization temperatureregion, and the surface carrying the layer is heated for bringing thetemperature of the material into its crystallization temperature region.

GENERAL ASPECTS OF THE INVENTION In the process of the presentinvention, crystallizing, powdered or granular polymer material is movedcontinuously through a chamber maintained at a pressure of less than onetorr with varying material layer depth and correspondingly changedforward velocity by vibrating heat transfer surfaces The layer thicknessand the temperature in various sections are selected so that nodisturbing agglomeration occurs when the material is heated into andabove the crystallization temperature region.

The process of the invention is based on the discovery that granular orpowdered synthetic polymer material of relatively low crystallizationtemperature region can be heated into and above its crystallizationtemperature region without there occurring agglomeration of theparticles, if the material layer depth is held below a predeterminedthickness.'When crystallization is finished, the layer thickness can beincreased for optimizing drying time and throughput.

The device of the present invention is based on the discovery of theeffect of layer depth on the drying of crystallizing, powdered orgranular synthetic polymers and, in general, on the principle that bothfaster drying times and a more protective treatment of a charge areeffected, when the product layer depth, that is to say the forwardingvelocity of the product layer and the temperature of the materialreceiving trays can be selectively and adjustably controlled. In otherwords, a faster and yet more tender thermal treatment of a powdered orgranular product can be obtained, when the product moving through thedevice is treated using a velocity and temperature program correspondingto the specific physical properties of the product.

The device of the invention includes a container and a plurality ofsubstantially circular treating trays arranged cascade-like one abovethe other in the container. The trays are singly or collectivelyheatable or coolable to varying temperatures. The forwarding of theproduct on the trays is accomplished by vibration in manner known perse. During transport around the axis of a tray, each particle of productmoves with the same angular velocity. When the material has beenforwarded to an outlet region of a tray, it falls down onto the inletregion of the next lower tray. According to the present invention, thecharacteristics of the vibrations are changeable for the purpose ofvarying the forwarding speed. In this way, substantial differences inthe residence times and layer depths on the individual trays can beobtained. Thus, the vibrational amplitudes of single trays or groups oftrays can be selected to this end. The throughput of the individualtrays, measured for example in kg/h, is a constant. Thus, for any twotrays l and 2 having different forwarding velocities V and V, anddifferent layer depths t, and t,, the forwarding velocities and layerdepths are related by the equation V, X V, X t:-

The device of the present invention makes it possible to subject amaterial to thermal treatment, as it moves continuously through cascadedtrays, under extremely varied conditions of temperature, layer depth,and velocity as selected in view of the particular properties of thematerial being treated. The forwarding of the material by vibration hasthe advantage that a good heat transfer is assured. Moreover, avibrational drive is capable of excellent control, so that adjustmentinto various vibration amplitudes for the treatment trays forinfluencing the velocity and layer depth of the material being treatedis possible in simple manner. Because the treatment conditions can bevaried in such wide ranges, it is possible to subject almost allflowable solids to thermal treatment in the device of the invention.

For the production of varying velocities for the material on the traysof the device, a number of vibrational drives can, for example, beprovided. Significantly more advantageous, however, is to provide onlyone vibrational drive, about midway in the height of the cascade, butoutside of the center of mass of the cascade. Then the upper and lowerparts of the cascade vibrate with different amplitudes, and this resultsin differing forwarding velocities for the powdered or granularmaterial. For the drying, crystallization, and solid condensation ofpolymers, the ability to obtain different material forwarding velocitiesis especially advantageous, since critical phases, such as suddenintensive heating or cooling, can be passed through with short residencetimes, while, for processes which physically of necessity require moretime, longer residence times can be chosen.

It is possible to arrange the vibrational drive substantially in thecenter of mass of the cascade. In this case, different vibrationalamplitudes can be produced for the upper and lower sections of thecascade by equipping one or the other of the sections with an auxiliarymass. By correctly choosing the size of the auxiliary mass and placingit in the proper one of the two sections, the residence times, andconsequently the layer depths of the material in the two cascade halvescan be varied at will.

To produce an especially effective action of the different thermalcharacteristics of the cascade on the material running through it, it isadvantageous to provide devices in the area of the treating trays forproducing an increased agitation in the powdered or granular material.Examples of such devices are a paddle wheel for continually scoopingmaterial out of the material layer and then pouring it back and a row ofjets directing pressurized gas into the layer to cause a temporaryfluidizing for the benefit of its mixing effect.

An intensive mixing of the charged material can also be obtained byarranging the treating trays like the steps of a helical staircase.Mixing occurs when the material falls from one tray to the next. Thesame effect can be obtained when the individual treatment trays arearranged parallel one above the other and a fall chute is positionedbetween the outlet region of an upper tray and the inlet region of thelower tray.

For the process of the present invention, the forwarding of the chargedmaterial by vibration has the advantage that an effective and uniformheat transfer is assured. Moreover, the ease with which vibrationaldrives can be controlled provides for good control of other processparameters. The product undergoes negligible mechanical wear and doesnot stick to the heat transfer surfaces. The form of the heat transfersurfaces can be chosen as desired.

The essential advantages of the process according to the invention ascompared with drying in a gyratory dryer lie in the significantlyshorter drying times and in the continuous operation. Space-consuming,temporary storage of material is no longer necessary. Moreover, comparedwith the fluidized-bed process, the present invention has the advantagesthat the uncontrollably different residence times of particles in thefluidized column are not present and that the drying of air oroxygen-sensitive polymers does not require special, expensive measures.

Mixing of the powdered or granular material as it is forwarded in theprocess of the present invention is advantageous for improving heattransfer from the surface on which the material is being forwarded intothe material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational, partiallycross sectional, partly schematic view of a device according to theinvention.

FIG. 2 is an elevational view of a modified portion of FIG. 1.

FIG. 3 is a cross sectional view from the line lII-III of FIG. 2.

FIGS. 4 to 6 are elevational views showing modified portions of FIG. 1.

FIG. 7 is an elevational partially sectional view of a modified portionof FIG. 1.

FIG. 8 is an elevational sectional view of a modified portion of FIG. 1.

' FIG. 9 is a perspective view of a modified portion of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The device of FIG. 1 includes acontainer 1, a cascade 2 mounted in the container, a section 3 forfeeding material into the container, and a section 4 for removingtreated material from the container. A vacuum is created in thecontainer 1 by a pump unit 5.

The cascade 2 includes a central pipe 6 and a vibrational drive 7interposed in the pipe about in the middle of the cascade but outside ofthe center of mass of the cascade. Vibrational drive 7 is anelectromagnetic vibratory drive made of at least two electromagneticvibration-producers and directing springs, which are so adjusted thatthe product is given the desired angle of throw. Such drives areavailable, for example, from the Vibration Technology Division of thefirm AEG, Niederrath-Frankfurt a.M., West Germany. Above and below thevibrational drive 7 are in each case four annular treating trays 8,groups I and II, secured to pipe 6. The floors 9 of these trays areprovided with conduits for conducting hot steam for the purpose ofcontrolling the floor temperatures. These conduits may be fedindividually or collectively according to the particular temperatureprogram desired for the trays. Thus, for purposes of illustration, setsof two trays 8 are shown connected to a common steam source 20 by abranched hose 21 containing control valve 25, it being apparent that anyof the hoses 21 could be branched to any numher of the trays and thatany suitable number of steam sources 20 of various temperature could beprovided for various ones of the trays. I-Ioses 21 hang somewhat looselyin container 1, in order to accommodate the rotational vibrations of thetray to which it is connected. Temperature may be selected byappropriate opening or closing of respective valves 25.

A second vibrational drive 30 is shown in FIG. 1 for attachment to groupII of the trays. When this drive 30 is connected to group II, group IIis disconnected from drive 7, so that groups I and II vibrate completelyindependently of one another. Both drives are substantially isolatedfrom container 1 by spring sets 31 and 32.

FIGS. 2 and 3 illustrate the provision of an auxiliary mass 10 in theinterior of pipe 6 for creating different vibration amplitudes in thetop and bottom parts of cascade 2. If the auxiliary mass 10 is below thevibrational drive 7, then the upper part of the cascade 2 vibrates witha larger amplitude than the lower part. If the auxiliary mass is above(dashed line representation, reference numeral 10'), then the lower partof the cascade 2 vibrates with a larger amplitude than the upper part.

Simple possibilities for providing a mixing of the powdered or granularmaterial while passing through the device of the invention are shown inFIGS. 4 and 5. The trays are all arranged horizontally and parallel toone another. The material moves on a circular path around one horizontaltreatment tray 8 in the direction of arrow '11 and then falls from theoutlet region of the tray obliquely in the direction of arrow 23 in FIG.4 and vertically in the direction of arrow 24 in FIG. 5 downwardly intothe inlet region of the next lower tray. This falling provides a goodmixing of the material.

In the embodiment of FIG. 6, the treatment trays 8 are mounted one afterthe other like the stairs of a helical staircase, and a continual mixingof the material occurs as the material falls from the outlet region ofone tray to the inlet region of the next lower tray. The effective pathof the material as it is forwarded by the rotational vibrations is shownby the arrows.

An intensive mixing of the charged material can be obtained in themanner of FIG. 7 by jetting gas into the material through the floors 9of the treatment trays 8. Thus, the floor 9 may be provided withopenings 12, while below these openings is an inert gas line 13 providedwith jets. The quantity of gas needed to provide a good mixing is sosmall, that a deterioration of the vacuum does not occur.

FIG. 8 shows a paddle wheel for agitating the material moving in thedirection of the arrows on a floor 9 by scooping into material on thefloor. The perspective view of FIG. 9 shows the paddle wheel and how itsaxis 15 is mounted in the outer side of the tray and in pipe 6. Meansfor rotating wheel 24 are not shown.

The right side of FIG. 9 illustrates an especially effectivemodification of the mixer of FIG. 8. The paddles are provided as spokes14' and these move between upstanding comb teeth 16 mounted in the floor9 of the tray 8.

FIG. 9 also illustrates in greater detail a typical tray of FIGS. 1, 2,and 4, with an inlet region 26, an outlet region 27, a conduit 22 in itsfloor 9 for the conduction of steam for controlling the floortemperature, a steam outlet hose portion 21' and a steam inlet hoseportion 21". In the process of the present invention, it is preferred touse layer depths I (see FIG. 8) lying between 1 and 8 centimeters, withthe smaller depths being used on the input side of the process. Thesedepths have been found to give the shortest drying times.

Further illustrative of the invention is the following example.

EXAMPLE I Granular polyester was dried in a vibratory dryer of theinvention having a total of 24 square meters of tray surface for heattransfer. The trays were separated into two groups of 15 each, each trayhaving an outer diameter D an inner diameter D,, and a breadth B, asshown in FIG. 9, equal, respectively, to. l ,200, 600, and 300mm. Theinterior of the container was. evacuated to a pressure of 0.1 torr. Thelayer depth t (see FIG. 8) was 2.5 centimeters in the upper group andcentimeters in the lower group. The temperature of the trays in theupper group increased from tray to tray from 100 to 140 C, while alltrays of the lower group were held at 160 C. The very last tray was usedfor cooling the product. For a terminal water content of 0.003weightpercent, a continuous throughput of 250 kilograms per hour couldbe attained. Mixing of the granular material occurred only when thematerial fell from an upper tray onto the next lower. An example of thepolyester is Polyester Co-Polymer of the firm lCI, having acrystallization temperature region from 70 to 120 C, an apparent densityaround 800 kg/m, and being in the form of chip-shaped granules of 4mmbreadth, 4mm length, and 2mm thickness.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

1. A device for the thermal treatment of powdered or granular materials,comprising a container, a plurality of substantially circular tray meansmounted one above the other in said container for the sequentialtransfer of material downwardly from an outlet region of one tray meansto an inlet region of a next lower tray means, means for selectivelycontrolling the temperatures of said tray means, and means for vibratingsaid tray means selectively and adjustably for moving material on saidtray means from their inlet regions to their outlet regions withpreselected material layer depths.

2. A device as claimed in claim 1, wherein said means for vibrating isprovided as a plurality of independent vibrational drives.

3. A device as claimed in claim 1, wherein said tray means are connectedto one another in a cascade and said means for vibrating comprises avibrational drive interposed about in the middle of said cascade butoutside of the center of mass of the cascade.

4. A device as claimed in claim 1, wherein said tray means are connectedto one another in a cascade and said means for vibrating comprises avibrational drive interposed about in the middle of said cascade,further comprising an auxiliary mass mounted to one half of saidcascade.

5. A device as claimed in claim 1, further comprising means for mixingmaterial on said tray means.

6. A device as claimed in claim 5, said means'for mixing comprising apaddle wheel means for scooping into material on said tray means.

7. A device as claimed in claim 5, said means for mixing comprisingmeans for jetting gas into material on said tray means.

8. A device as claimed in claim 1, wherein said tray means are mountedone after the other like the stairs of a helical staircase.

9. A device as claimed in claim 1, said tray means being arrangedparallel to one another, one above the other, further comprising meansfor allowing material to fall from the outlet region of an upper traymeans onto the inlet region of a next lower tray means.

10. A device as claimed in claim 1, wherein said tray means compriseannular trays.

11. A device as claimed in claim 1, wherein said means for temperaturecontrol comprises controllable means for heating said tray means withsteam.

2. A device as claimed in claim 1, wherein said means for vibrating isprovided as a plurality of independent vibrational drives.
 3. A deviceas claimed in claim 1, wherein said tray means are connected to oneanother in a cascade and said means for vibrating comprises avibrational drive interposed about in the middle of said cascade butoutside of the center of mass of the cascade.
 4. A device as claimed inclaim 1, wherein said tray means are connected to one another in acascade and said means for vibrating comprises a vibrational driveinterposed about in the middle of said cascade, further comprising anauxiliary mass mounted to one half of said cascade.
 5. A device asclaimed in claim 1, further comprising means for mixing material on saidtray means.
 6. A device as claimed in claim 5, said means for mixingcomprising a paddle wheel means for scooping into material on said traymeans.
 7. A device as claimed in claim 5, said means for mixingcomprising means for jetting gas into material on said tray means.
 8. Adevice as claimed in claim 1, wherein said tray means are mounted oneafter the other like the stairs of a helical staircase.
 9. A device asclaimed in claim 1, said tray means being arranged parallel to oneanother, one above the other, further comprising means for allowingmaterial to fall from the outlet region of an upper tray means onto theinlet region of a next lower tray means.
 10. A device as claimed inclaim 1, wherein said tray means comprise annular trays.
 11. A device asclaimed in claim 1, wherein said means for temperature control comprisescontrollable means for heating said tray means with steam.